A Java program which is written in Java language just can be run in computer which has installed the Java Platform.

Java Language defines the syntax and semantics of the Java programming language. It defines basic vocabulary such as primitive types, if/else blocks, the syntax of class declaration, exception syntax, variable scoping rules, and everything else necessary for the language to function.

What is Java API?

Java API or Java ‘Application Programming Interface‘ : are libraries of compiled code that you can use in your programs. They let you add ready-made and customizable functionality to save you programming time. The Java API is the set of classes included with the Java Platform bundle. For example, the list of API classes in Java 2 Platform Standard Edition V 1.4.2 can be seen at http://java.sun.com/j2se/1.4.2/docs/api/

What is Java Virtual Machine (JVM)?

As I said before that our Java Source-Code must be compiled becoming –> “Byte-Code” ….. then a program which is linking one or Byte-Codes can be run (interpreted) on JVM.

Rather than running directly on the native operating system, the program is interpreted by the Java VM for the native operating system. This means that any computer system with the Java VM installed can run Java programs regardless of the computer system on which the applications were originally developed. For example, a Java program developed on a Personal Computer (PC) with the Windows NT operating system should run equally well without modification on a Sun Ultra workstation with the Solaris operating system, and vice versa.

So..the JVM executes Java bytecode. Normally, you produce Java bytecode by compiling code that conforms to the Java language. (However, you can compile code written in other languages into Java bytecode.)

Again, make sure before you can write and run the simple Java program in this lesson, you need to install the Java platform on your computer system!!!